1. Field of the Invention
The present invention relates to a liquid crystal driving power supply circuit for generating a plurality of power supply voltages having different values used to drive a liquid crystal display panel.
2. Description of the Related Art
Liquid crystal display panels have low power consumption and a small size. Owing to these advantages, the liquid crystal display panels are used as the display units of portable electronic devices such as electronic desk calculators and electronic pocketbooks. In order to drive such a liquid crystal display panel, a plurality of voltages having different values are required. These liquid crystal driving voltages are generally formed by a voltage dividing operation using a plurality of resistors arranged between power supplies. The maximum value of a current which can be caused to flow from the node of each voltage formed by such a voltage dividing operation is determined by the value of each of the plurality of resistors. Therefore, the amount of current flowing from the node of each voltage may be increased by decreasing the value of each resistor. In this case, however, a large amount of current flows between the power supplies, resulting in an increase in power consumption. On the other hand, in order to decrease this power consumption, the value of each resistance may be increased to decrease the amount of current flowing between the power supplies. In this case, however, the amount of current which can be caused to flow from the node of each voltage decreases. If a large current flows from each node, the voltage of each node decreases, and the value of each voltage cannot be maintained at a specified value.
In general, in order to solve the above problem, a resistor having a high resistance is used as each voltage dividing resistor to achieve a reduction in power consumption, and each divided voltage is received by a power amplifier to achieve a reduction in the impedance of an output. The power amplifier has a differential input stage and an output stage. The differential input stage receives the respective divided voltages described above and an output voltage from the output stage connected to the differential input stage. The output stage has a constant-current source and a driving transistor to which an output from the differential input stage is supplied. Sufficiently large currents need to be supplied to the constant-current source and the driving transistor on the output stage to maintain an output voltage at a predetermined value even if a large current flows from or into the power amplifier. Consequently, the power consumption of the power amplifier increases, and the effect obtained by using a resistor having a high resistance as each voltage dividing resistor is reduced. This results in shortening the service life of a battery in a portable electronic device which is driven by the battery.